In inductive circuits, when vacuum switches are used in specific cases, e.g. in turning off a starting motor, multiple re-ignitions may occur with virtual current ruptures, which lead to a strong voltage load in the incoming turns of the switched equipment and which may possibly require protective measures. See, e.g., K. Stegmuller, Elektrotechnik 66/22, Nov. 1984, pp. 16-23. Therefore, there is a need for an overvoltage-free vacuum switchgear which does not have this tendency of voltage increases when switching small currents in inductive circuits. This means there is the requirement for the contact material in such switches to have a long arc burning to the range of zero current, i.e., after a low rupture current of approximately less than 0.2 A, and at the same time after a sufficiently conductive arc, so as to reduce the instability of the rupture process to a minimum. To fulfill this requirement, charge carriers must be generated in sufficient number by the arc during switching, i.e., a high rate of vapor generation from the cathode must exist.
The switch-off capacity of the system is imperiled by the intensive delivery of metal vapor and hence by a large quantity of charge carrier. Therefore, a contact material is required which shows an overvoltage-free switching behavior as well as a high power switching capacity.
To obtain contact materials with an overvoltage-free switching behavior, it has been previously proposed to add to a base material, e.g. CuCr, a sufficient quantity of readily vaporizable additive components. Such materials are described, for example, in EP-A No. 00 83 200, EP-A No. 00 83 245, U.S. Pat. No. 4,424,429 and EP-A No. 00 90 579.
The additives discussed in these references are already known for use in vacuum switch contact materials for other purposes, for example, for rupture current reduction or welding force reduction, and are distributed largely homogeneously in the volume of the contact material by various methods, so as to be always replenishable in case of burnoff losses.
These known practices have serious disadvantages.
Since with an increasing magnitude of the switch-off current the amount of vaporized contact material and hence of charge carriers increases greatly due to the presence of the readily vaporizable additives, the switch-off capacity is impaired considerably with increasing currents and is clearly reduced in comparison with materials without additives.
Due to the high percentage of, as a rule, brittle additive materials or brittle phases of these materials, the material loses its necessary ductility, which is important for the mechanical load during the switching process and for good electrical contact-making under permanent current load.
At the same time, due to the poorly conducting additive materials, the current and heat conduction of the electrodes is reduced, i.e., problems resulting from increased evolution of heat may occur.
Such contact material combinations are preferably produced by powder metallurgical techniques. Because of the production technique and in consideration of the additives used, e.g., as with the material disclosed in U.S. Pat. No. 4,424,429, a considerable susceptibility to structure defects, inhomogeneities and high contents of residual gases results which bring about an additional limitation in switching efficiency and in voltage strength.
A major obstacle in terms of fabrication by the use of the mentioned type of the so-called "low surge" contact materials results from the fact that most of the cited additives also have good anti-welding properties. Materials highly alloyed with these additives may create considerable problems with respect to their bonding technology.